1. Field of the Invention
The present invention relates to display intensity control apparatus for display systems and more particularly to a control apparatus for controlling the display intensity of automobile electronic display systems under dark ambient conditions.
2. Description of the Prior Art
Recent development of display elements and electronic circuit components has led to the use of light-emitting display elements such as fluorescent display tubes (hereinafter referred to as FLT's) or light-emitting diodes (hereinafter referred to as LED's) in the various meters and clocks on automotive vehicles.
Where such light-emitting type display elements are used, the display intensity must be increased under light ambient conditions such as the daytime outdoor conditions and the display intensity must be decreased under dark ambient conditions such as the nighttime outdoor conditions. Generally, under light ambient conditions, the display elements are continuously energized to increase the display intensity (the intensity obtained by continuous energization of display elements is hereinafter referred to as a daytime intensity). Under dark ambient conditions the display elements are energized and deenergized repeatedly to flash on and off so fast that the human eye cannot perceive this fact and the duty ratio of the flashing of the display elements is varied to control the average intensity at a suitable value (the average intensity determined according to the duty ratio of the flashing is hereinafter referred to as a nighttime intensity). On the other hand, generally, in many case of automobile meters employing FLT's or LED's it is also effected to illuminate the meter dials with an incandescent lamp (hereinafter simply referred to as a lamp) and the illumination by the lamp must be controlled in harmony with the intensity of the FLT's or LED's. In this specification, a combination of a dial and a lamp is also considered as a display element. Thus, the term, display element intensity, also referes to the intensity of such lamp.
While the intensity of the FLT or LED decreases substantially in proportion to the flashing ratio (the ratio of the off-time to on-time), the intensity of the lamp varies non-linearly with the flashing ratio. Also, while the daytime intensity of the FLT is considerably high making it necessary to decrease the intensity with a high flashing ratio under dark ambient conditions, the daytime intensity of the LED is not so high that it becomes excessively dim if its intensity is decreased at the same rate with the FLT under dark ambient conditions. As a result, it is difficult to effect the desired display intensity control under dark ambient conditions of a single combination meter or of a plurality of successively arranged individual meters (e.g., a speedometer, a tachometer, a fuel gage and a thermo-gage) with a single signal source (a rheostat). However, in view of limitations in cost, difficulties in operation, etc., it is not practical to provide display intensity controlling rheostats for the FLD's, LED's and lamps, respectively, so as to adjust their intensities individually.